Raw material gaseous feedstocks such as hydrogen and oxygen can be formed by the electrochemical dissociation of water. Fuel cells and secondary batteries (e.g., rechargeable batteries) can be used to combine or recombine hydrogen and oxygen to create electricity. Gaseous products such as hydrogen are also suitable for use as a gaseous fuel in many types of combustion engines. Other uses and vast markets exist for such gaseous products and raw materials.
Fuel cell and secondary battery technology has been hampered by a number of problems concerning the generation and storage of hydrogen or hydrogen containing compounds, and oxygen and oxygen containing compounds. Gaseous H2 is extremely volatile and combustible, so storage of all but the smallest quantities of the gas is fraught with flammability and safety concerns. Generation and storage of gaseous hydrogen at moderate temperatures also requires the use of high-pressure generation and containment equipment. This equipment is extremely expensive and cumbersome to use. Generation and storage at moderate pressure requires the use of low temperatures and substantial refrigeration. Cooling requirements such as these are costly and substantially reduce the energy efficiency of such processes. In addition, the processes of generating and pressurizing degrade the quality of the produced hydrogen and oxygen gas. In cases where high purity hydrogen gas or its isotopes are required, additional costly manufacturing steps are required to remove contaminates.
Although hydrogen gas has a high energy density, its associated storage requirements result in a substantially reduced achievable energy per unit weight ratio. This issue in particular dramatically reduces the practicality and energy efficiency of hydrogen consuming fuel cells, secondary batteries, and engines, especially for mobile applications.
The generation and storage of oxygen and or oxygen containing compounds has had a similar analogous impact on the practicality and energy efficiency of fuel cells, secondary batteries, and engines, especially for mobile applications.
Certain fuel cell applications require three-phase gas diffusion electrodes to be in simultaneous contact with the gaseous fuel (typically hydrogen), the solid electrically-conductive portion of the electrode, and the electrolyte. The same requirement is placed upon an oxygen three-phase gas diffusion electrode. Typically, fuel cell oxygen is derived from ambient air. To operate efficiently the oxygen should simultaneously contact the solid electrically-conductive portion of the electrode, and the electrolyte. This design limitation of these types of fuel cells adversely affects their versatility and utility.
A further problem encountered by certain secondary battery and fuel cell applications involves the storage of hydrogen in metal hydrides. The amount of hydrogen that can be stored in these hydrides is less than 2% by weight. Commercial metal hydrides are now available. For example, metal hydride alloys (sometimes known as AB5 hydrogen-absorbing compounds) are capable of storing about 1 gram of hydrogen per 65.5 grams of material, or only about 1.5% by weight.
A further problem encountered by certain fuel cell applications involves pressure differential problems largely caused by gravity in large-scale electrodes. As is well known in fuel cell art, a fuel cell can be used to combine hydrogen and oxygen gases to form water and electricity. The gas feedstocks are generally introduced on opposite sides of the fuel cell with a liquid electrolyte disposed between them. The electrolyte and gas feedstocks are generally separated by a porous electrode, which may include a membrane. In a common configuration, a porous electrode separates the electrolyte from the hydrogen supply, and another porous electrode separates the same electrolyte from the oxygen supply. The electrolyte is thus disposed between and in part contained by the two porous electrodes.
For economy of scale, it is generally desirable to increase the physical height of such fuel cell electrodes. As the height of these types of fuel cells is increased, the differential pressure across each porous electrode increases due to the difference in head pressure caused by the significantly different densities of the liquid electrolyte and feedstock gases on opposite sides of the porous electrode. The magnitude of the pressure differential that can be withstood by the electrode poses a practical limit to the height of such fuel cells and thus limits their overall capacity, efficiency, and scale. Horizontal orientation of such fuel cells results in other related problems. The pressure gradient developed down the electrolyte side of the porous electrode results in variable gas migration rates of each feedstock gas into the electrolyte. For a given gas supply pressure, the gas migration rate through the porous electrode and into the electrolyte is greater near the top of the vertical porous electrode than near the bottom, due to the reduced hydraulic backpressure present at the higher elevation. This non-uniform gas flow through the electrode and into the electrolyte hampers the efficiency of the electrochemical cell, since not all portions of the electrode experience the same amount of gas flow. These problems have not been fully overcome and they apply equally to both the hydrogen and oxygen gas diffusion electrodes. Also, for fuel cells in which the electrolyte is circulated by external pumps, the electrolyte pressures between the electrodes can be much higher than the static pressure head alone. In these cases, the performance of the gas diffusion electrode can be even more greatly impacted by pressure differential or balancing problems.
Further problems with certain types of secondary batteries such as metal hydride batteries relate to their limited energy (hydrogen) storage capacity. The capability to store additional energy within such batteries could reduce the number of required charging cycles and also allow them to provide energy output for extended periods of time.
What is needed is an apparatus and method for forming and efficiently storing high purity hydrogen gas (or its isotopes) that makes this produced gas easily and effectively available for later use as, for example, a fuel for combustion, as a feedstock to a fuel cell or secondary battery, or as a feedstock for nuclear applications. The hydrogen needs to be storable in a safe, lightweight, low pressure, moderate temperature apparatus that readily makes it available for use, without prolonged liberation times.
What is further needed is an apparatus and method for forming and efficiently storing high purity oxygen gas that makes this produced gas easily and effectively available for later use as, for example, an oxidizer for reaction or combustion, or as a feedstock to a fuel cell or secondary battery. The oxygen needs to be storable in a safe, lightweight, low pressure, moderate temperature apparatus that readily makes it available for use, without prolonged liberation times.
What is further needed is such an apparatus and method that eliminates the differential pressure problems encountered in many fuel cell applications. What is further needed is a way to increase the energy stored per unit weight ratio for fuel cells and secondary batteries.